Plastic laminates have been useful for a variety of products because of their relative light weight, good strength characteristics, resistance to corrosion, and low cost. A variety of methods have been developed for making laminated structures according to a pre-determined shape.
One particularly useful method is rotational molding, in which the laminate is made even while the molded structure is being formed. Rotational molding is advantageous in the manufacture of large structures such as boat hulls and vehicle bodies.
A typical rotational molding apparatus for making a molded laminated structure, such as a boat hull, would include a mold assembly having means to rotate about two axes, an oven into which the mold assembly can travel and in which it can "rock and roll" about the two axes. The apparatus would have one or two insulated boxes positioned to release particulate plastics into the mold cavity while it rocks and rolls in the oven.
The molding method carried out with this apparatus would include the following steps: First, the insulated boxes would be filled with the appropriate plastic particulates, chosen to provide the desirable laminate qualities. For example, one of the boxes could be filled with the appropriate charge of a plastic with a blowing agent, while the other could have a plastic without a blowing agent. Then, another charge of plastic particulate material would be placed loosely in the mold cavity, which would then be closed.
The mold assembly would be moved into the oven and the rocking and rolling action would begin. The oven would heat the mold assembly and mold, eventually causing the mold cavity wall to reach a temperature causing melting of the particulates sliding thereacross to start forming a layer in contact with the cavity wall. This layer would get thicker as the rotational movement continues, and cross-linking of the material would begin.
At some point before cross-linking is completed, one of the insulated boxes would be opened to release the particulate plastic and blowing agent. Melting of the new material would begin as it contacted the hot outer layer, and the new material would form as a second layer intimately bonded to the first layer. The second layer would become a foam layer by virtue of the action of the blowing agent.
After substantial completion of the formation of the foam layer, but before completion thereof, the other insulated box would be opened to release the remaining charge of particulate plastic. This material would begin to melt by virtue of the heat of the foam layer, and would imtimately bond with the foam layer and continue to coalesce to form an inside layer.
After coalescence of the inside layer, the mold assembly would be removed from the oven and cooling would begin. Cooling may be carried out by use of fans blowing on the mold assembly, which may continue its rotational movement to allow the fanned air to contact the outer surfaces of the mold relatively evenly. During cooling, while the molded piece remains in the mold, the mold cavity may or may not be opened by opening doors exposing the mold cavity to the cooling air.
After cooling, the molded piece is stripped from the mold, and the process can begin again.
There are many variations possible in rotational molding, including the use of other heating methods, other cooling methods, and many different types of equipment. Many advances have been made in rotational molding, and a number of U.S. patents have been issued, including: 3,936,565, 3,391,823, 3,663,680, 4,247,279, 4,022,564, 3,419,455, 3,813,462, 3,703,571, 3,527,852, 3,914,361, 3,962,390, 3,843,285, 3,505,137, 3,541,192, 2,893,057, 3,134,140, 3,822,980, 3,676,037, 3,703,348, 3,810,727, 3,825,395, 3,841,821, 3,885,016, 3,914,105, 3,997,649.
There are certain problems or shortcomings with rotational molding methods, particularly when used to mold various large items such as boat hulls. For example, warping can occur during cooling in the mold, particularly if there is relatively uneven cooling as is the case when a mold cavity is opened to allow faster cooling. In such cases, the outer surface of the molded piece, which is against the mold wall, will cool more slowly than the inner surface of the molded piece, which is exposed to the cooling air when the mold cavity is opened for cooling.
One possible solution to this is to keep the mold cavity closed during cooling such that the difference in the rate of cooling is not as great as it is when the cavity is opened. However, this means that the cooling cycle will be lengthened, delaying the time the the mold will be available for molding the next piece.
Another very significant shortcoming with certain types of rotational molding methods, such as that described above, is the large amount of heat which is required. The amount of heat required is very substantial for two reasons--the oven is very large to accommodate the rocking and rolling mold assembly and the time required for the process is lengthy. If the laminate has several layers, as may be necessary to achieve the desired properties, the time necessary for the process is extended.
Shortening the in-oven cycle time would yield substantial savings in energy costs and makes the rotational molding equipment more efficient in terms of output of pieces per unit of time.
Substantial improvements have been needed to address the problems and drawbacks of rotational molding discussed above.